1. Field of the Invention
The present invention relates to a voltage detecting device for a set battery.
2. Description of Related Art
Detection of a remaining capacity of a set battery mounted on an electric vehicle or the like is generally carried out based on an open-terminal voltage estimated from total voltage and current thereof. Further, in a set battery of an electric vehicle, several hundreds of cell batteries are connected in series to generate high voltage. As a result, loss of wiring resistance is normally reduced. It is required that a voltage detecting device detects with high accuracy a total voltage of the set battery in which a number of battery cells are connected in series for generating electric power for an electric vehicle running while achieving reduction in power consumption and simplification in circuit constitution. However, meeting such a requirement is not easy. For example, assuming that voltage detection accuracy of a voltage detecting circuit is 0.1%, error of 0.3 V is caused when a total voltage of a set battery is 300 V. Meanwhile, a terminal voltage of a battery cell is about 1.2 V and accordingly, it is difficult to check failure of each battery cell and/or charging state by the total voltage.
Further, the detected total voltage is normally A/D (Analog to Digital)--converted for signal processing in a microcomputer. However, if the resolution of an A/D convertor is 11 bits, a resolution error of about 0.15 V is caused. Accordingly, an expensive and highly accurate A/D convertor is obliged to adopt, and necessary detection accuracy is not achieved with an inexpensive A/D convertor having a low resolution. Further, a semiconductor device having a high withstand voltage is needed to detect the high total voltage and accordingly, expense therefor must be disbursed.
According to a charging apparatus for a set battery disclosed in Japanese Unexamined Patent Publication No. JP-A-5-64377, module voltages of a plurality of modules (each module is constituted at least by one cell battery) constituting a set battery in which a number of cell batteries are connected in series, are individually detected. Charge control of the set battery is carried out with respect to the respective modules based on the detected module voltages.
That is, as shown in FIG. 5, a set battery is divided into a plurality of battery modules 101 through 120 and module voltages are detected by voltage detecting circuits 201 through 220, respectively. By summing up these module voltages, a total voltage of the set battery is calculated and a charge state of each of the battery modules is grasped.
However, when a differential-type voltage detecting circuit is used as a voltage detecting circuit for detecting each of the module voltages, there arise problems explained below.
Each of the differential-type voltage detecting circuits 201 through 220 includes an operational amplifier, an input side resistor circuit for setting both input terminals' potential (imaginal ground potential) and a feedback resistor. A lower potential terminal of the input side resistor circuit is connected to a lower potential side power source terminal and thereby the imaginal ground potential is set to a predetermined reference potential. Thereby, currents (hereinafter, referred to as input signal currents) flowing into or flowing out of signal input terminals of each differential voltage detecting circuit 201 through 220 can be considered to flow into the lower potential side power source terminal via the input side resistor circuit.
The battery modules 101 through 120 supplies current to the differential voltage detecting circuits 201 through 220, respectively, for detecting the respective module voltages.
Conventionally, the input terminal imaginal ground potentials of the operational amplifiers are uniformly set to a potential at the low potential side power source terminal of the set battery 100. Accordingly, the magnitude of the respective input signal current charged by each battery module in the battery modules 101 through 120 significantly differs. As a result, a voltage detection error and a dispersion in battery consumption are caused.
A further detailed explanation will be given in reference to FIG. 5 as follows.
Input signal current is fed from a high potential terminal of the battery module 101 to a negative input terminal of the differential voltage detecting circuit 201. The current flows to the low potential side power source terminal of the set battery 100 via the input side resistor circuits. After all, current is fed from all of the battery modules 101 through 120 to the negative input terminal of the differential voltage detecting circuit 201. Further, the same goes with a positive input terminal of the differential voltage detecting circuit 201.
The input signal current is fed from a high potential terminal of the battery module 102 to a negative input terminal of the successive differential voltage detecting circuit 202. The current flows to the low potential side power source terminal of the set voltage 100 via the input side resistor circuits. After all, current is fed from all the battery modules 102 through 120 excluding the battery module 101 in the respective battery modules 101 through 120 to the negative input terminal of the differential voltage detecting circuit 202.
Current is fed similarly in the successive differential voltage detecting circuits. Input signal current is fed from a high potential terminal of the battery module 120 to a negative input terminal of the differential voltage detecting circuit 220. The current flows to the low potential side power source terminal of the set battery 100 via the input side resistor circuit of the differential voltage detecting circuit 220. After all, current is fed only from the battery module 120 to the negative input terminal of the differential voltage detecting circuit 220.
As a result, it is known that the battery module 101 at the highest potential feeds the input signal current only to the differential voltage detecting circuit 201 and the battery module 120 at the lowest potential supplies the input signal current to all of the respective differential voltage detecting circuits 201 through 220.
Products of the input signal currents and internal resistances of the respective battery modules constitute internal voltage drops of the battery modules, respectively, which lower the module voltages of the battery modules 101 through 120. In this case, by the operation as mentioned above, some of the battery modules (for example, the battery module 120 or battery modules at a vicinity thereof) supply the input signal currents to a number of the differential voltage detecting circuits. Therefore, the lowering of the module voltage becomes significant and power consumption thereof is also increased.
Further, high voltage is applied on the input side resistor circuit of the differential voltage detecting circuit on a higher potential side and therefore, power consumption thereof is increased. When the resistance value of the input side resistor circuit is set high to restrain the power consumption, thermal resistance noise is increased and the frequency characteristic is deteriorated.
Further, in the conventional device, high withstand voltage needs to be assured in respect to resistors of the input side resistor circuits of the differential voltage detecting circuits on the higher potential side.